In conventionally known motor control units for controlling motors of compressors, an AC voltage from an AC power supply is rectified by a rectifier circuit to become a rectified voltage having rectified pulses, and the rectified voltage is hardly smoothed and input to an inverter circuit. An example of such motor control units is disclosed by Patent Literature 1.
Specifically, the motor control unit of Patent Literature 1 includes a single-phase diode full-wave rectifier circuit, a low-capacity smoothing capacitor and a PWM control inverter. The low-capacity smoothing capacitor is a capacitor having a capacity of about 1/100 of that of a smoothing capacitor of a conventional motor control unit. Therefore, in practical, the motor control unit of Patent Literature 1 does not have a capacitor for keeping the voltage constant. A fall-wave-rectified voltage rectified by the single-phase diode full-wave rectifier circuit is hardly smoothed and input to the PWM control inverter in the waveform as shown in FIG. 7. The PWM control inverter receives the rectified voltage and outputs a polyphase AC current to the motor. The motor control unit of this kind allows reduction in size and cost as the capacity of the capacitor is low.
As disclosed by Patent Literature 2, another known motor control unit performs torque control to vary an output torque in response to variation in load torque that occurs in a single rotation of the motor of the compressor. In the case of a rotary compressor, for example, a load torque of the motor periodically varies in the course of fluid compression including a set of suction, compression and discharge processes. Therefore, if an output torque of the motor is constant, the rotation speed of the compressor varies and the compressor vibrates. The torque control is performed to reduce the vibration of the compressor.
Patent Literature 1: Published Japanese Patent Application No. 2002-51589
Patent Literature 2: Published Japanese Patent Application No. 02-17884